Power amplifiers account for a significant portion of the capital and operational expense in current wireless base station designs. One method of reducing this expense is to increase the overall efficiency of these amplifiers. In order to obtain high-efficiency amplifiers, one prior approach is to modulate the amplifier's PSU (power supply unit) in order to track the input envelope of the modulated signal. This in turn requires that one must have a high-efficiency PSU which supports the desire to modulate the PSU, while at the same time, does not introduce any impairments into the main signal path.
A number of high-efficiency architectures have been proposed, particularly in the audio field. However, these architectures typically consist of a single phase and are not capable of supporting the power levels and input bandwidths required for RF applications.
Improvements in efficiency of power amplifiers would benefit low-frequency, high-frequency and radio-frequency amplifier applications, ultimately providing for increased flexibility as to base station placement, particularly when efficiencies increase to the point where supplemental heat dissipation via fans and the like is no longer required.
Sigma-Delta modulation allows noise shaping such that the noise of the modulated signal lies mostly out-of-band. Filtering off the out-of-band noise substantially restores the original signal. See for example Sharp “An Overview of Sigma-Delta Converters”, (IEEE Signal Processing Magazine, January 1996), SM-SX1 Sigma-Delta Audio Amplifier (IEEE Spectrum, March 2000), “Bandpass delta-sigma class-S amplifier”, Electr. Letters, Vol. 36, No. 12, June 2000, “Linear High-Efficiency Microwave Power Amplifiers Using Bandpass Delta-Sigma Modulators”, Jayaraman et al., IEEE Micr. & Guided Wave Letters, Vol. 8, No. 3, March 1998, “Linear Amplification by Sampling Techniques: A new application for Delta Coders”, Cos, IEEE Trans. Comm., Vol. Com-23, No. 8, August 1975. Conventional applications for Sigma-Delta modulation have focused on analog inputs, and have produced single-bit outputs only. Single bit systems require a very high over-sampling rate to achieve acceptable performance. Multi-bit Sigma-Delta modulators have also been proposed. Sigma-Delta modulation takes an input signal and converts it to an N-level quantized Sigma-Delta signal. The input signal can be in the form of an analog signal or a digital signal.
Various methods and apparatus for efficiently amplifying or converting a signal, including high-efficiency amplifiers and methods, are disclosed in United States Patent Application Publication No. 2004/0239416 (Ser. No. 10/449,105 filed on Jun. 2, 2003 and published on Dec. 2, 2004) and United States Patent Application Publication No. 2005/0062526 (Ser. No. 10/858,079 filed on Jun. 2, 2004 and published on Mar. 24, 2005), each of which is incorporated herein by reference.
In most high efficiency applications, filters are utilized to reject signals in particular frequency ranges, outside a desired passband. Conventional filter implementations typically introduce dissipation into the signal processing path. Other components or processing blocks or network within the path may also increase dissipation. Therefore, there is a need for filter design and implementation (and other component design and implementation) that reduces dissipation and results in increased efficiency.